Heat transfer for thermal management between two materials at different temperatures often may be accomplished by conduction, radiation and/or convection. In the area of electronics, in a narrow region at, for instance, an interface between a die lid (e.g., commonly a copper-tungsten material) of the integrated circuit and the heat sink, the temperature present in the integrated circuit (IC) can typically be between about 40° C. to 150° C. For such a situation, thermal management may typically be accomplished through conduction. However, the use of flat plates at the interface to facilitate the heat transfer from the integrated circuit to the heat sink has not been optimal. In particular, the use of a flat plate may provide only between 20 to 50 points of contact to the integrated circuit and/or the heat sink. As a result, the heat that flows out of the hot integrated circuit can only pass through these few contact spots.
To enhance the transfer of heat to the heat sink, current technology usually involves placing a thermally conducting grease between the die lid of an integrated circuit and the heat sink device. The heat sink device, in general, may be of any type, including a passive heat sink, a Peltier cooler, a refrigerated copper block, a heat pipe, or an active fan type, or a copper block in which embedded heat pipes can carry heat to a water-cooled bus outside of the system.
Presently, thermal greases that are commercially available typically contain silver powder or silver flake, and may be used by applying to machined, and occasionally, lapped heat sinks and integrated circuit lids. However, the thermal conductivity of these commercially available greases at best may only be about 9 watts/m-deg K. For example, (i) Arctic Silver III has a thermal conductivity of >9.0 W/m-deg K, (ii) AOS Thermal Compounds has a thermal conductivity of about 7.21 W/m-deg K, (iii) Shin-Etsu G751 has a thermal conductivity of about 4.5 W/m-deg K, (iv) AOS Thermal Compounds HTC-60 has a thermal conductivity of about 2.51 W/m-deg K, (v) Thermagon T-grease has a thermal conductivity of about 1.3 W/m-deg K, and (vi) Radio Shack Thermal Grease has a thermal conductivity of about 0.735 W/m-deg K. As illustrated in FIG. 1, there exists, generally, a 20 degrees difference between the heat source and the heat sink. Such a difference may indicate a thermal resistance at the junction and suggests that the potential to carry heat to the sink may be hurt by the poor interface provided by the grease.
It has been known that metal fiber structures and material can provide a low loss connection at greatly reduced forces, thereby providing high-efficiency, low force electrical contact. Based on simple laws of physics, the capability of fiber brushes to efficiently transfer electrical current across interfaces, which can be in relative motion or at rest, is paralleled by their capability to similarly transfer heat. In particular, since they operate at low loads and have very low resistance, they can dissipate relatively much less heat. Moreover, the fiber brushes can provide a substantial amount contact points between the heat source and heat sink to permit efficient heat transfer. As a result, metal fiber brushes have been used in a thermal interface as heat conduits for cooling or heating purposes. (U.S. Pat. No. 6,245,440)
Recently, carbon nanotubes have been used in thermal management. It has been shown that the thermal conductivity of carbon nanotubes is over 2980 watts/m-deg K as compared to thermal grease, which is only about 9 watts/m-deg K maximum (Thermal Conductivity of Carbon Nanotubes by Jianwei Che*, Tahir Cagin, and William A. Goddard III Materials and Process Simulation Center California Institute of Technology Pasadena, Calif. 91106 E-mail: jiche@caltech.edu. Even higher numbers are reported by Tománek (VOLUME 84, NUMBER 20 PHYSICAL REVIEW LETTERS 15 MAY 2000 “Unusually High Thermal Conductivity of Carbon Nanotubes,” Savas Berber, Young-Kyun Kwon,* and David Tománek).
In addition, U.S. Pat. No. 6,891,724, discloses the use of carbon nanotubes deposited on a CVD diamond coated thermally heat die. In particular, a CVD diamond coating is placed on a heat die, and the die subsequently coated with carbon nanotubes.
In Carbon nanotube composites for thermal management, M. J. Biercuk, M. C. Llaguno, M. Radosavljevic, J. K. Hyun, and A. T. Johnson, Department of Physics and Astronomy and Laboratory for Research on the Structure of Matter, University of Pennsylvania, Philadelphia, Pa. 19104—Applied Physics Letters—Apr. 15, 2002—Volume 80, Issue 15, pp. 2767-2769, the authors discussed adding a small amount of carbon nanotubes, without surface modification, to an epoxy matrix to improve heat transfer.
In Study of Carbon Nanofiber Dispersion for Application of Advanced Thermal Interface Materials, Xinhe Tang*, Ernst Hammel, Markus Trampert, Klaus Mauthner, Theodor Schmitt, Electrovac GmbH, Aufeldgasse 37-39, 3400. Klosterneuburg, Austria and Jürgen Schulz-Harder, Michael Haberkorn, Andereas Meyer, Curamik Electronics GmbH, Am Stadtwald 2, 92676 Eschenbach, Germany, the authors described how adding carbon nanotubes to thermal grease improves thermal performance.
Others have developed approaches to aligning nanotubes in arrays for other applications. For example, Jung, Y. J., et al. “Aligned Carbon Nanotube-Polymer Hybrid Architectures for Diverse Flexible Electronic Applications.” Nano Lett., 6 (3), 413-418, 2006, discloses a nanotube filled polymer but does not include thermal applications.